A polymer electrolyte fuel cell (PEFC) includes a necessary number of stacked single cells, each including a catalyst layer bonded to an electrolyte membrane for a power generation reaction between separators. Fuel gas and oxidizer gas are supplied to the solid polymer-type fuel cell so as to electrochemically react through the electrolyte membrane, thereby simultaneously generating electric power, heat, and water.
The outer periphery of the electrolyte membrane is mainly supported by a resin frame to which a fixed seal called gasket is injection-molded so as to prevent leakage of the fuel gas. The electrolyte membrane fixed to the frame is sandwiched between the separators so as to form the single cell.
In recent years, the electrolyte membrane is a proton conductive ion exchange membrane. A positive ion-exchange membrane composed of a perfluorocarbon polymer having a sulfonic group is particularly excellent in basic properties and thus has been widely examined. Actually, a required electrolyte membrane for a solid polymer-type fuel cell has a low ohmic loss.
A solid polymer-type fuel cell has the following reactions:H2→2H++2e− on the negative electrode, and½O2+2H++2e−→H2O on the positive electrode.These reactions generate electrical energy. The electric resistance of a positive ion-exchange membrane is controlled by the mobility of protons in the positive ion-exchange membrane. The larger the water content in the positive ion-exchange membrane, the higher the mobility of protons. This reduces the electric resistance of the membrane. The positive pole is kept at a high water content because water is generated by the reaction, leading to high mobility of protons, whereas the negative electrode relatively has a low water content and thus it is assumed that the water content limits the mobility of protons of the ion-exchange membrane.
The electric resistance of the positive ion-exchange membrane is reduced by increasing the concentration of a sulfonic group and reducing the thickness of the membrane. However, a considerable increase in the concentration of a sulfonic group may reduce the mechanical strength of the membrane or cause the creep of the membrane during a long run of a fuel cell, disadvantageously reducing the durability of the fuel cell.
Moreover, an electrolyte membrane containing a high concentration of a sulfonic group considerably swells due to contained water and thus may cause various problems. The membrane dimensions are increased by steam or the like supplied with water generated during power generation or fuel gas. The increased dimensions of the membrane may cause “wrinkles” filling the grooves of separators so as to interfere with a gas flow.
Furthermore, repeatedly started and stopped operations cause the membrane to repeatedly swell and shrink. This may cause cracks on the membrane and electrodes joined to the membrane, leading to deterioration of battery characteristics.
As methods for solving the problems, the insertion of a reinforcement into an electrolyte membrane and the provision of films stacked with different water contents have been proposed (Patent Literatures 1 and 2).
In Patent Literature 1, an electrolyte membrane contains nonwoven fibers of a polyvinylidene fluoride polymer as a reinforcing material.
FIG. 9 is a schematic diagram of a conventional electrolyte membrane.
In FIG. 9, a fuel cell stack includes at least two films containing perfluorocarbon polymers with different water contents, and has a water easily containing structure in which a film 41 that increases in water content toward the negative electrode is located. This prevents a reduction in the mobility of protons on the negative electrode of a membrane and reduces an electric resistance. A film 42 having a low water content on the positive pole has the function of increasing the strength of the membrane (Patent Literature 2).